JP2002105604A - High Cr martensitic stainless steel pipe for line pipe excellent in corrosion resistance and weldability and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high Cr martensitic stainless steel pipe for line pipe excellent in corrosion resistance and weldability and a method for producing the same. SOLUTION: C: 0.02% or less, N: 0.07% or less, Ni:
0.2 ~7.0%, Mo: 0.2~3.0% , Cr: comprises 10 to 14%, Si, Mn, P, the steel pipe was within the proper range S, and heated to Ac ₃ transformation point or above the temperature, and then air cooled After cooling at the above cooling rate to obtain a quenched structure, tempering is performed at a temperature of 520 ° C. or more to precipitate a γ phase, thereby obtaining a martensite structure containing a γ phase having an area ratio of 5% or more. Furthermore, Ti: 0.15%
In the following, Nb: 0.2% or less, Zr: 0.15% or less, V: 0.2% or less, Ta: 0.15% or less selected from one or more, and Ca: 0.006% or less. Is also good.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-Cr martensitic stainless steel pipe suitable for use in line pipes for transporting petroleum and natural gas, and more particularly to improvement in corrosion resistance and weldability.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of soaring crude oil prices and the depletion of oil and natural gas resources expected in the near future, deep oil fields and oil and gas fields which have not been seen in the past and which are highly corrosive have not been seen. Etc. are being actively developed. Such oil and gas fields generally at high depths, situated in the so-called remote areas such as marine and cold land, also carbon dioxide CO _2, chlorine ions Cl ^- an increasing number where has a severe corrosive environments containing such I have.

In such a highly corrosive wet carbon dioxide gas environment, if carbon steel is used as a material for oil country tubular goods or line pipes, it is significantly corroded. For this reason, as a means of preventing corrosion of oil well pipes and line pipes, inhibitors have been added to mined crude oil and gas. However, the addition of inhibitors is expensive because the inhibitors are expensive, and the effect of adding the inhibitors is insufficient at high temperatures.In recent years, oil well pipes and line pipes have been used instead of the addition of inhibitors. There is a tendency to use corrosion-resistant materials as a material for the sapphire.

[0004] As such a corrosion resistant material, in oil country tubular goods,
Martensitic stainless steel containing 13% Cr has been widely used. Also, recently, in order to adapt to a corrosive environment containing a small amount of hydrogen sulfide, for example, Japanese Patent Application Laid-Open No. 60-174859 discloses a method of adding Ni, Mo, etc., to a martensitic stainless steel containing 13% of Cr, Oil well pipes with improved SSC properties have been proposed. However, the oil country tubular goods described in Japanese Patent Application Laid-Open No. Sho 60-174859 have no consideration given to weldability, and have a problem that welding cracks occur when welding is performed without preheating and postheating.

On the other hand, as a material for line pipe, AP
The I standard prescribes a 12% Cr martensitic stainless steel pipe with a reduced C content. However, since this kind of steel pipe has low weldability, it requires preheating and post-heating at the time of welding, lowers the efficiency of welding work, increases cost, and has the disadvantage of low weld toughness, and has hardly been used. Was. In place of this kind of material, a duplex stainless steel excellent in weldability and corrosion resistance has been used as a line pipe material. However, duplex stainless steel has a large amount of alloying elements and has excessive performance depending on the place of use, which may be economically expensive.

Further, since high-temperature gas / fluid flows in the line pipe, it is necessary to keep the strength (high-temperature strength) of the line pipe in use. In order to increase the strength (high-temperature strength) of the line pipe during use, it is customary to take measures such as increasing the room-temperature strength of the line pipe to increase the high-temperature strength or increasing the thickness of the line pipe. is there. However, if the room-temperature strength of the line pipe is increased, the weldability may be degraded, and if the thickness of the line pipe is increased, the material cost may increase.

[0007]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems of the prior art, and has sufficient overall corrosion resistance and pitting corrosion resistance in a corrosive environment containing carbon dioxide gas. Excellent SSC resistance in environments containing hydrogen sulfide
It is an object of the present invention to propose a high Cr martensitic stainless steel pipe for line pipes having excellent corrosion resistance and weldability, exhibiting excellent weld heat affected zone toughness, and a method for producing the same.

[0008]

Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors have studied the corrosion resistance of a high Cr martensitic stainless steel tube in a corrosive environment containing carbon dioxide gas, and have found that welding cracks and welding heat Various factors affecting the toughness of the affected zone were studied diligently. As a result, C and N contents were made appropriate contents, Ni and Mo were contained, and a structure containing austenite of 5% or more was obtained.
It has been found that, by combining the above, a martensitic stainless steel pipe having excellent corrosion resistance and weldability even under a corrosive environment containing carbon dioxide gas can be obtained for the first time. It has been found that this results in a steel pipe with excellent SSC resistance and toughness in the HAZ.

The present invention has been completed based on the above findings and further studies. That is, in the first invention, in mass%, C: 0.02% or less, Si: 1.0% or less,
Mn: 0.2 to 3.0%, P: 0.05% or less, S: 0.005% or less, Cr: 10 to 14%, Ni: 0.2 to 7.0%, Mo: 0.2 to 3.0
%, Al: 0.1% or less, N: 0.07% or less, having a composition comprising the balance of Fe and unavoidable impurities and a structure containing a martensite phase as a main phase and an austenite phase with an area ratio of 5% or more. This is a high Cr martensitic stainless steel pipe for line pipes having excellent corrosion resistance and excellent weldability. In the first aspect of the present invention, in addition to the above-described composition, in addition to the above components, Ti: 0.15% or less, Nb: 0.2% or less, Zr: 0.15% or less, V: 0.2% or less, Ta: 0.15% or less. It is preferable to contain one or more selected from the above, and in the first aspect of the present invention, it is preferable to further contain 0.006% or less by mass% of Ca in addition to the above-mentioned respective compositions. .

[0010] The second present invention relates to a method for producing C: 0.1% by mass.
02% or less, Si: 1.0% or less, Mn: 0.2 to 3.0%, P: 0.
05% or less, S: 0.005% or less, Cr: 10-14%, Ni: 0.2
-7.0%, Mo: 0.2-3.0%, Al: 0.1% or less, N: 0.
Including 07% or less, or in addition, Ti: 0.15% or less, N
b: 0.2% or less, Zr: 0.15% or less, V: 0.2% or less, T
a: using a steel material containing one or more selected from 0.15% or less, and preferably comprising a balance of Fe and inevitable impurities, to form a steel pipe having a predetermined size, The steel pipe was heated to a temperature not lower than the Ac ₃ transformation point, then cooled to obtain a quenched structure, and then tempered at a temperature of 520 ° C. or more to precipitate an austenite phase.
A method for producing a high Cr martensitic stainless steel pipe for line pipes having excellent corrosion resistance and weldability, characterized by having a structure containing a martensite phase as a main phase and an austenite phase having an area ratio of 5% or more, In the second aspect of the present invention, in addition to the above composition, Ca: 0.1% by mass.
It is preferred to contain 006% or less.

[0011]

First, the reasons for limiting the composition of the steel pipe of the present invention will be described. Hereinafter, mass% is simply described as%. C: 0.02% or less C is an element that increases the strength of the base metal, but increases the hardness of the heat affected zone, increases the susceptibility to weld cracking, and reduces the toughness of the heat affected zone. It is desirable to reduce it. In order to improve corrosion resistance such as pitting corrosion resistance in a corrosive environment containing carbon dioxide, it is desirable that C be low. In the present invention, in order to enable welding without preheating, C is set to 0.02% or less. In addition, preferably 0.01%
It is as follows.

Si: 1.0% or less Si acts as a deoxidizing agent and has an effect of increasing strength. To achieve this effect,
It is desirable to contain 0.1% or more. Note that Si is a ferrite forming element, and when contained in a large amount, ferrite is generated, and the toughness of the base material and the weld heat affected zone deteriorates. For this reason, Si was limited to 1.0% or less. Incidentally, preferably 0.
1 to 0.5%.

Mn: 0.2-3.0% Mn is an element that acts as a deoxidizing agent and increases strength. Further, Mn is an austenite-forming element, suppresses the formation of ferrite, and improves the toughness of the base metal and the heat affected zone. Such an effect is recognized at a content of 0.2% or more. However, if the content exceeds 3.0%, the effect is saturated and an effect corresponding to the content cannot be expected, so that it is economically disadvantageous. For this reason, Mn is limited to the range of 0.2 to 3.0%. The content is preferably 0.5 to 2.0%.

P: 0.05% or less P increases the strength, but decreases ductility and toughness, and further deteriorates corrosion resistance. In particular, segregation at grain boundaries lowers grain boundary strength, adversely affecting SSC resistance. Therefore, it is desirable to reduce P as much as possible. However, an extreme reduction leads to a rise in manufacturing costs. For these reasons, the present invention is limited to a range of 0.05% or less, which is industrially relatively inexpensive and does not extremely deteriorate toughness and corrosion resistance. In addition, it is preferably 0.03% or less.

S: 0.005% or less S is an element that forms sulfide-based inclusions such as MnS and remarkably deteriorates hot workability. It is desirable that S be reduced as much as possible to improve productivity. However, an extreme reduction leads to a rise in manufacturing costs. If S is reduced to 0.005% or less, production can be performed in a usual process. Therefore, in the present invention, S is limited to 0.005% or less. Incidentally, preferably 0.
003% or less.

Cr: 10 to 14% Cr is an element that forms a martensitic structure to increase the strength, forms a protective film, and increases corrosion resistance such as pitting corrosion resistance in a corrosive environment containing carbon dioxide gas. To obtain such an effect, Cr needs to be contained at 10% or more. On the other hand, if the content exceeds 14%, the tendency of ferrite formation becomes strong, and a large amount of austenite-forming elements must be contained in order to stably secure the martensitic structure, which is expensive and economically disadvantageous. . For this reason,
In the present invention, Cr is limited to the range of 10 to 14%.

Ni: 0.2-7.0% Ni is an austenite-forming element, and has an effect of increasing strength and toughness and suppressing a decrease in strength and a decrease in toughness due to a decrease in C and N. Ni strengthens the protective coating and increases corrosion resistance such as pitting corrosion resistance in a corrosive environment containing carbon dioxide gas. Further, Ni has an effect of suppressing the generation of δ-ferrite at a high temperature and enhances the hot workability of the Mo-containing steel. In order to obtain such effects, a content of 0.2% or more is required. However, a content of more than 7.0% is economically disadvantageous because a large amount of expensive Ni is added. Therefore, in the present invention, Ni is limited to the range of 0.2% to 7.0%. Incidentally, the content is preferably 0.5 to 5.5%.

Mo: 0.2-3.0% Mo is an element that improves quenching properties and increases strength, and also has the effect of improving SSC resistance. To obtain such an effect, the content of 0.2% or more is required. However, when the content exceeds 3.0%, ferrite is easily formed, the strength toughness is reduced, and the effect of improving the SSC resistance is reduced. descend. For this reason, Mo is limited to 0.2 to 3.0%. Preferably, the content is 0.5 to 2.5%.

Al: 0.1% or less Al acts as a deoxidizing agent, but if it exceeds 0.1%, toughness is reduced. Therefore, in the present invention, Al is limited to 0.1% or less. In addition, it is preferably 0.05% or less. N: 0.07% or less N, like C, forms a solid solution in steel and increases the strength of the base metal, increases the hardness of the weld heat affected zone, increases the weld cracking susceptibility, and improves the toughness of the weld heat affected zone. Element that lowers
In the present invention, it is desirable to reduce as much as possible. From the viewpoint of welding cracking, up to 0.07% is acceptable, so in the present invention, N is limited to 0.07% or less. Preferably,
0.03% or less.

Ti: 0.15% or less, Nb: 0.2% or less, Zr: 0.
One or more of Ti, Nb, Zr, V, and Ta selected from 15% or less, V: 0.2% or less, and Ta: 0.15% or less. It has the effect of improving the strength toughness and the toughness of the weld heat affected zone, and can be selected and contained as needed. Also, Ti,
Nb, Zr, V, and Ta increase the effective Cr amount with respect to pitting corrosion resistance by replacing Cr carbide with carbides of these elements, thereby improving pitting corrosion resistance. Ti: 0.15%, Nb: 0.2%,
If the content exceeds Zr: 0.15%, V: 0.2%, Ta: 0.15%, the susceptibility to weld cracking increases, the risk of weld cracking increases, and the toughness of the base metal and the weld heat affected zone deteriorates. It is preferable that these values be the upper limits of the respective elements. In addition, more preferably, Ti: 0.01 to
0.1%, Nb: 0.01-0.1%, Zr: 0.01-0.1%, V: 0.
01 to 0.1%, Ta :: 0.01 to 0.1%.

Ca: 0.006% or less Ca has the effect of forming sulfide CaS, suppressing the formation of easily soluble MnS, and improving the corrosion resistance. However, a large content causes inclusions in the form of clusters to be formed, and reduces the toughness of the base material. For this reason, Ca is preferably limited to 0.006% or less.

The balance Fe and inevitable impurities The balance other than the above components is Fe and inevitable impurities. Incidentally, O: 0.01% or less can be tolerated as inevitable impurities. The structure of the steel pipe of the present invention has a martensite phase as a main phase and a structure containing an austenite phase at an area ratio of 5% or more. The main phase referred to in the present invention is an area ratio of 50
% Means the phase which occupies more than%. By containing 5% or more of austenite in martensite which is a main phase, precipitates of Cr, precipitates of Mo, and the like are reduced, corrosion resistance is improved, and the strength of the steel pipe can be reduced by short-time heat treatment.
On the other hand, if the austenite phase is less than 5%, the corrosion resistance is reduced due to the large amount of Cr and Mo precipitates. The more the austenite phase, the more preferable from the viewpoint of corrosion resistance. However, if it exceeds 35%, there is a problem that it becomes difficult to obtain a predetermined strength. Therefore, the austenite phase is 5% or more,
Preferably it is 35% or less.

As described above, the structure of the steel pipe of the present invention has a main phase of martensite, contains austenite, and further contains precipitates of 3% (area ratio) or less. There is no problem even if ferrite is contained at 3% (area ratio) or less. Next, a method for producing the high Cr martensitic stainless steel pipe for a line pipe of the present invention will be described.

First, the molten steel having the above-described composition is produced by a commonly known method such as a converter or an electric furnace, and is made into a steel pipe material such as a billet by a continuous casting method or a steel ingot-bulk rolling. preferable. These steel pipe materials are subjected to a normal pipe forming process, that is, heating, drilling with a Mannesmann drilling machine, hot rolling using an inclined rolling mill such as a plug mill method, a mandrel method, etc. to obtain a seamless steel pipe of a predetermined size. Is preferred.
In the present invention, it is needless to say that the present invention is not limited to a seamless steel pipe, and a welded steel pipe such as an electric resistance welded steel pipe, a UOE steel pipe, or a spiral steel pipe may be used.

The electric resistance welded steel pipe is formed, for example, into a steel strip by hot rolling a steel pipe material having the above-described composition, and is formed into an electric resistance welded steel pipe having a predetermined size according to a normal pipe forming process, that is, a forming-welding-straightening process. Is preferred. The UOE steel pipe and the spiral steel pipe are not particularly limited in the pipe forming process, and it is preferable to form a pipe according to a normal UOE steel pipe manufacturing step and a spiral steel pipe manufacturing step to obtain a steel pipe having a predetermined size.

It is manufactured by the above-mentioned ordinary pipe forming process,
The steel pipe having the above-described composition and having a predetermined size is heated to a temperature equal to or higher than the Ac ₃ transformation point, and then cooled at a cooling rate equal to or higher than air cooling to obtain a quenched structure. If the heating temperature of the steel pipe is lower than the Ac ₃ transformation point, the heating temperature is too low to obtain a perfect austenite structure and a sufficient hardened structure. Here, the sufficient quenched structure means a structure having quenched martensite in an area ratio of 95% or more. On the other hand, 1050
If the temperature exceeds ℃, the austenite grains become coarse and the toughness is deteriorated.

After the steel pipe is heated, it is cooled to room temperature at a quenching cooling rate higher than air cooling. As the cooling, air cooling, mist cooling, water cooling and the like are all suitable. After quenching, the steel pipe is 52
Tempered at temperatures above 0 ° C. The tempering conditions (temperature and time) are preferably such that an austenite phase is formed at 5% or more. If the tempering temperature is lower than 520 ° C., a long-time tempering is required to form an austenite phase of 5% or more, and productivity is impaired. Therefore, the tempering temperature is preferably at least 520 ° C., more preferably at most the Ac ₁ transformation point. A more preferred tempering temperature is 600 to 650 ° C.

[0028]

EXAMPLES Steel having the composition shown in Table 1 was melted in a converter, subjected to vacuum degassing and refined, and then made into a steel pipe material (billet) by a continuous casting method. These steel pipe materials are heated and made into a pipe by a Mannesmann-mandrel mill.
A seamless steel pipe having a size of 273 mm and a wall thickness of 12.7 mm was used. Then, these steel pipes were subjected to heat treatment (quenching-tempering) under the conditions shown in Table 2.
To obtain an X80 grade steel pipe.

Test specimens were taken from these steel pipes, and were examined for structure, tensile test, impact test, and corrosion test, and evaluated for strength, toughness, and corrosion resistance. In addition, the test piece was sampled in the as-quenched state, and the quenched structure was examined. (1) Microstructure Investigation The cross section perpendicular to the longitudinal direction of each steel pipe was imaged with a light microscope or a scanning electron microscope, and the area ratio of each phase was measured using an image analyzer. The amount of austenite phase (γ) in the structure was determined by X-ray diffraction.
It was calculated from the ratio of the diffraction intensity of γ from (220) to the diffraction intensity of α from (211). (2) Tensile test A round bar test piece of 6 mmφ was taken from the longitudinal direction of each steel pipe, a tensile test was performed, and the yield strength YS, tensile strength TS, and elongation El were measured. (3) Impact test A JIS No. 4 test piece was sampled from the longitudinal direction of each steel pipe, an impact test was performed at a test temperature of -40 ° C, and the Charpy absorbed energy vE- ₄₀ was measured. (4) Corrosion test Carbon dioxide corrosion test Specimens (size: 3.0 x 25 x 50 mm) taken from each steel pipe
Was immersed for 7 days in a 25 mass% aqueous NaCl solution (liquid temperature: 100 ° C.) saturated with 3.0 MPa of carbon dioxide gas in an autoclave, and then pulled up. After removing the corrosion product from the pulled-up test piece, the presence or absence of pitting corrosion was visually inspected. Thereafter, the weight of the test piece after the corrosion test was measured and converted into the amount of reduction in sheet thickness to obtain the corrosion rate (mm / y).

From these results, the pitting corrosion resistance was evaluated as "O" when pitting occurred and "X" when no pitting occurred. In addition, the corrosion rate: 0.127mm / y as the limit value,
The sample having a corrosion rate equal to or greater than the limit value was evaluated as x, and the sample having a corrosion rate less than the limit value was evaluated as ○. SSC test Using a test piece (size: 6.4 mmφ) collected from each steel pipe, a constant load test was performed in accordance with the provisions of NACE-TM 0177 method A, and the SSC resistance was evaluated. The test liquid is 5 mass% Na
Cl + 0.5 mass% CH ₃ COOH aqueous solution, CH ₃ COONa is added
The pH was adjusted to 3.5. In addition, 1% H ₂ S + 99% CO
_The test was performed while blowing the mixed gas of No. ₂ . The load stress was 90% YS, and the test time was 720 hours. YS
Used the yield stress (654 MPa) of the lower limit of X80.

Also, two steel pipes (length: 0.5 m) of the same type are prepared, and the ends (V groove processing) are joined to each other.
A steel pipe joint was produced by circumferential welding using GMAW welding.
The produced steel pipe joints were examined for the occurrence of weld cracks. Circumferential welding conditions: heat input: 19.5 kJ / cm
(Voltage: 14.5 V, current: 157 A, welding speed: 7.0 cm / mi
n) The GMAW welding method was used, and no preheating and afterheating were performed.

In addition, welding cracks were investigated by observing the cross section of welding. ○: No welding cracks occurred, ×: Cracking occurred
And the weld cracking property was evaluated. In addition, a Charpy impact test specimen (JIS No. 4 specimen) was sampled from the weld heat affected zone (HAZ) (1 mm from the bond) of the steel pipe joint, and the test temperature was −
The Charpy absorbed energy vE- ₄₀ at 40 ° C was determined.

Table 2 shows the results.

[0034]

[Table 1]

[0035]

[Table 2]

Each of the examples of the present invention has excellent strength and toughness, can be circumferentially welded without performing preheating and post-heat treatment, has excellent weld cracking resistance, and has excellent weld heat affected zone toughness. Excellent pitting resistance and overall corrosion resistance even in corrosive environments including SS, and in corrosive environments containing hydrogen sulfide
It can be seen that the steel pipe has excellent C properties. On the other hand, in Comparative Examples outside the scope of the present invention, one or more of the above-mentioned characteristics are deteriorated.

[0037]

According to the present invention, line pipes having excellent corrosion resistance even in a corrosive environment containing carbon dioxide gas or a corrosive environment containing hydrogen sulfide and capable of circumferential welding without requiring preheating and post heat treatment. High Cr martensitic stainless steel pipes can be easily manufactured at low cost and have a remarkable industrial effect. Further, according to the present invention, a pipeline for transporting oil and natural gas can be manufactured at low cost.

Continuing from the front page (72) Inventor Takaaki Toyooka 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki Steel Corporation Chita Works (72) Inventor Murase Fumio Murase 1-1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Kawasaki Steel Corporation F term in the Chita Works (reference) 4K042 AA06 BA06 BA11 CA03 CA07 CA08 CA09 CA10 CA11 CA12 CA13 CA16 DA01 DA02 DC02

Claims (6)

[Claims] 1. In mass%, C: 0.02% or less, Si: 1.0% or less, Mn: 0.2 to 3.0%, P: 0.05% or less, S: 0.005% or less, Cr: 10 to 14%, Ni: 0.2 -7.0%, Mo: 0.2-3.0%, Al: 0.1% or less, N: 0.07% or less, the composition comprising balance Fe and unavoidable impurities,
A high-Cr martensitic stainless steel pipe for line pipes having excellent corrosion resistance and weldability, characterized by having a structure containing a martensite phase as a main phase and an austenite phase having an area ratio of 5% or more. 2. The composition according to claim 1, further comprising:
i: 0.15% or less, Nb: 0.2% or less, Zr: 0.15% or less,
V: 0.2% or less, Ta: 0.15% or less
2. The composition according to claim 1, wherein the composition contains at least two species.
High-Cr martensitic stainless steel pipe for line pipes as described in (1). 3. The composition according to claim 1, further comprising:
2. The composition according to claim 1, wherein a: 0.006% or less.
Or a high Cr martensitic stainless steel pipe for a line pipe according to item 2. 4. In mass%, C: 0.02% or less, Si: 1.0% or less, Mn: 0.2 to 3.0%, P: 0.05% or less, S: 0.005% or less, Cr: 10 to 14%, Ni: 0.2 ~7.0%, Mo: 0.2 ~3.0% , Al: 0.1% or less, N: After a steel pipe of predetermined size and forming tube using a steel material having a composition comprising 0.07% or less, the steel pipe, Ac ₃ After heating to a temperature higher than the transformation point and then cooling to obtain a quenched structure, tempering is performed at a temperature of 520 ° C. or higher to precipitate an austenite phase, a martensite phase as a main phase, and an area ratio of 5% or more. A method for producing a high Cr martensitic stainless steel pipe for line pipes having excellent corrosion resistance and weldability characterized by having a structure containing an austenitic phase. 5. The composition according to claim 1, further comprising:
i: 0.15% or less, Nb: 0.2% or less, Zr: 0.15% or less,
V: 0.2% or less, Ta: 0.15% or less
5. The composition according to claim 4, wherein the composition contains at least one species.
The method for producing a high-Cr martensitic stainless steel pipe for a line pipe according to the above item. 6. The composition according to claim 1, further comprising:
a: 0.006% or less.
Or the method for producing a high Cr martensitic stainless steel pipe for a line pipe according to 5 above.